The present invention relates to liquid crystal display devices, and, more particularly, to liquid crystal display devices of the lateral electric field type.
A liquid crystal display device of the lateral electric field-driven type comprises a pair of transparent substrates disposed so as to be opposed to each other and to be spaced by a liquid crystal (LC) layer sealed therebetween, and a pixel electrode and a counter electrode spaced apart from the pixel electrode are formed in each of a matrix of pixel regions on a surface at the liquid crystal layer side of one of the pair of transparent substrates. The pixel electrode and the counter electrode disposed at each pixel region generate an electric field therebetween so as to control the optical transmissivity of the liquid crystals disposed in the vicinity of the pixel region. Since the liquid crystal display device of the lateral electric field type generates an electric field having a major component extending substantially parallel to the main surface of the transparent substrate between the pixel electrode and the counter electrode for switching orientations of the liquid crystal molecules, the lateral electric field scheme is also referred to as in "In-Plane-Switching (IPS) scheme".
An active-matrix liquid crystal display device employing the lateral electric field scheme has a plurality of scan signal lines and a plurality of counter voltage signal lines, both of which extend in an "x" direction (row direction), while being juxtaposed in parallel in a "y" (column) direction (transverse to the x direction), and a plurality of video signal lines, which extend in the y direction, while being juxtaposed in the x direction, on the surface at the liquid crystal layer side of the one of the pair of transparent substrates. When the scan signal lines, the counter voltage signal lines and the video signal lines are disposed on a surface of one of the pair of transparent substrates, each of the pixel regions is defined as an area surrounded by one of the scan signal lines, one of the counter voltage signal lines adjacent to the one of the scan signal lines, and a pair of the image signal lines disposed adjacent to one another.
Each of the pixel regions has a switching element, a pixel electrode, and a counter electrode. The switching element operates so as to be turned on by a scanning signal which is supplied by one of the scanning signal lines. The switching element, when turned on, supplies a video signal, which is being transmitted by one of the video signal lines thereto, to the pixel electrode in the pixel region. A counter voltage for generating a potential difference between the counter electrode and the pixel electrode in the pixel region is transmitted through one of the counter voltage signal lines and is applied to the counter electrode.
A liquid crystal display device having the structure described above is characterized by an image having a higher contrast ratio and a remarkably wider viewing angle. The lateral electric field-type liquid crystal display device is described in detail in such documents as Japanese Patent Application Laid Open Hei 05-505247 (No. 505247/1993), Japanese Patent Publication Sho 63-021907 (No. 021907/1988), and Japanese Patent Application Laid Open Hei 06-160878 (No. 160878/1994).
According to a recent trend toward widening the screen size of a liquid crystal display device, the wiring resistance of each of the signal lines in the liquid crystal display device is required to be lower. However, in complying such a requirement, a decline in the yield factor for production of the liquid crystal display device due to the signal line being disconnected (open circuit) has been experienced.
In seeking a cause of such disconnection of the signal lines, the inventors have found that this problem results from the configuration (of a laminated structure) of the liquid crystal display device, in which a portion (crossing portion, hereinafter) where one of the video signal lines crosses over one of the scanning signal lines is spaced from the one of the video signal lines by an insulating film. More specifically, when the liquid crystal display device has a laminated structure including a conductive layer forming the scanning signal line (a first conductive layer, hereinafter), an insulating film, and another conductive layer forming the video signal line (a second conductive layer, hereinafter) being stacked on (over) one of the pair of transparent substrates in this order, a bump due to the thickness of the first conductive layer appears on an upper surface of the insulating film in the area of the crossing portion. Both the first and the second conductive layers are formed of thin films of such material as a metal, an alloy, an oxide conductor, and the like, and the second conductive layer tends to be fabricated so as to be thinner than the insulating film. Although the first conductive layer is thinner than the insulating film, the bump becomes so much higher (thicker) on the upper surface of the insulating film that the second conductive film being formed on the upper surface of the insulating film is broken by the bump at the crossing portion, and the video signal line becomes disconnected. Even if the scanning signal line is formed of the second conductive film and the video signal line is formed of the first conductive film, the same problem also occurs at the crossing portion, with the scanning signal line being disconnected in this case.
On the other hand, after the video signal lines are formed, another insulating film called a protective film is formed so as to cover even the video signal line. Since the second conductive film of the video signal line is disposed over the first conductive film of the scanning signal line in the crossing portion, another bump appears on the upper surface of the second conductive film (the video signal line). The inventors have found that the protective film PSV is cracked by this other bump so that the second conductive film of the video signal line is eroded by the chemicals (reagent chemicals, solvent, etc.) used for the process of fabrication of the liquid crystal display device, infiltrating through the crack to the second conductive film. This erosion problem also occurs when the scanning signal line is formed of the second conductive film and the video signal line is formed of the first conductive film.